A conventional ultrasound imaging system comprises an array of transducer elements for transmitting an ultrasound beam and receiving a reflected ultrasound beam from the object being studied. By selecting the phase delay and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the phase rotation and amplitude of the input voltages provided to the transducer elements, the ultrasound beam may be moved to scan the object.
The same principles apply when the array is employed to receive the reflected ultrasound energy. A receive beamformer typically focuses the transducer elements in the array on a focal point while receiving ultrasound energy. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate phase delay and gain to the signal from each of the receiving transducer elements.
When acquiring ultrasound data, such as in B-mode imaging, conventional ultrasound imaging systems typically transmit an ultrasound beam focused at a single focal point positioned along the line. Then, the transducer elements detect reflected ultrasonic signals. After transmitting the ultrasound beam, the transducer elements are used to detect samples of the reflected ultrasound beam at different points in time. Acquiring each of the samples may use some or all of the transducer elements. Additionally, each of the samples corresponds to a different time or depth along the line. The transducer elements of a conventional ultrasound system typically convert the ultrasound energy into electrical signals. The electrical signals, in turn, are sent to a receive beamformer where the appropriate phase delays and gains are applied to each of the electrical signals in order to “focus” the transducer array on the correct depth for the received ultrasound signal. The beamformer typically adjusts the focus of the array so that the transducer elements are focused on the appropriate depth for the sample being acquired. After beamforming, the electrical signals acquired at a particular point or sample are combined into a signal indicative of the acoustic reflectivity of the object at a specific point along the line. In order to generate an image, a processor typically maps the amplitude of signals from the beamformer to a gray scale for display on a monitor or other display device.
One problem with conventional ultrasound imaging systems is that they are particularly sensitive to electromagnetic noise. Any external or internal electromagnetic noise may alter electric signals from the transducer elements and/or the beamformer. If left uncorrected, electromagnetic noise may cause artifacts in ultrasound images. For example, external electromagnetic noise which is coherent across channels, often results in images with central regions showing increased pixel intensity. This region of increased pixel intensity is sometimes referred to as a “flashlight artifact” because the region of brightness resembles the beam of a flashlight. The effects of random noise, or noise that changes significantly over a short period of time, is harder to characterize, but might still result in images with pixel values that are not indicative of the received ultrasound signal.
Conventional ultrasound imaging systems have taken steps to isolate the ultrasound imaging systems from electromagnetic noise. For instance, they have employed various types of shielding, including Faraday cages, in an attempt to minimize the electromagnetic noise in the ultrasound imaging system. However, such isolation techniques are typically less than perfect and, in noisy environments, significant electromagnetic noise may still penetrate into the system. Electromagnetic noise may be particularly problematic when a strong electromagnetic source, such as an RF, or Bovie, knife, is operating is close proximity to the ultrasound imaging system.
For these and other reasons there is a need for an improved method and ultrasound probe for ultrasound imaging.